Switched-capacitor, common-mode feedback circuit for a differential amplifier without tail current

ABSTRACT

Provided is a switched capacitor feedback circuit including two or more input ports configured to receive a corresponding a number of input signals and at least one output port. The output port is configured to output an adjusting signal. The input signals includes a number of primary signals and two or more reference signals that are associated with a first timing phase of operation. The adjusting signal is produced based upon a comparison between the primary signals the reference signals. Also provided is a pair of active devices having gates coupled together and structured to receive the adjusting signal. The active devices are configured to provide a gain to the adjusting signal in accordance with a predetermined gain factor, and facilitate an adjustment to the number of primary signals based upon the gain during a second timing phase of operation.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application No. 60/223,280 filed Aug. 3, 2000, and U.S. Provisional Application No. 60/224,168 filed Aug. 9, 2000, which are both incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention generally relates to a circuit for a differential amplifier. More particularly, the present invention relates to a system and method for controlling output-common-mode voltages in differential amplifiers without tail currents.

2. Background Art

An important advantage in transitioning to finer-geometry complementary metal oxide (CMOS) processes is the ability to use low-voltage power supplies, which corresponds to a lower-power operating environment. Although CMOS processes fulfill the need for low-power operating requirements and consumption, a need still exists to be able to achieve large signal swings from these CMOS amplifiers in order to maintain adequate signal-to-noise ratios. Several conventional approaches to providing differential amplifiers with reasonably large output swings exist. A few of these traditional approaches are discussed in greater detail below.

Several metrics exist for rating and measuring the overall performance of differential amplifiers. Among these metrics are total current, slew current, and output swing. As known in the art, differential amplifiers include an inverting input and a non-inverting input and corresponding inverting and non-inverting outputs. The total current for differential amplifiers, therefore, is measured by adding the absolute value of the current detected at both the inverting and non-inverting outputs.

Slew current is defined as the amount of current detected at one amplifier output port when completely tilting one input to the other. Typically, slew current is used as a measure of an amplifier's efficiency. Another important metric is the output swing, which is a measure of how far the amplifier can go from rail to rail before it begins losing gain. Although a number of different techniques exist for providing large signal output swings in CMOS amplifiers, each of these techniques suffers from at least one major disadvantage.

For example, while certain amplifiers may be able to produce large output swings, these same amplifiers may be slow or inefficient. Other amplifiers that may be faster, typically sacrifice efficiency for speed. Finally, other amplifiers that provide reasonably large output swings and high efficiency, may be unable to reject output common-mode voltage disturbances. Common-mode voltage, as understood in the art, can be created as a result of imbalances in transistor performance and power supply variations. Common-mode voltage can also originate from the input source directly or from a previous amplification stage.

Although particular types of differential amplifiers provide tail current sources as an effective technique for resolving problems associated with common-mode voltages, tail current sources create inefficiencies and inherently reduce headroom swing in these amplifiers. Therefore, a need exists to be able to derive high output swings from low-power power supplies in CMOS differential amplifiers without tail current sources.

BRIEF SUMMARY OF THE INVENTION

Consistent with the principles of the present invention as embodied and described herein, an embodiment of the invention includes a switched capacitor feedback circuit configured to switch between two or more timing phases of operation. The circuit includes (i) a plurality of input ports configured to receive a corresponding plurality of input signals there through, and (ii) at least one output port configured to output an adjusting signal. Next, the corresponding plurality of input signals includes a number of primary signals and two or more reference signals. The adjusting signal is produced based upon a comparison between the number of primary signals and at least one of the reference signals, the comparison occurring during a first of the timing phases. Finally, the primary signals are adjusted in accordance with the adjusting signal during a second of the timing phases.

Another embodiment of the invention includes a switched capacitor feedback circuit including (i) a plurality of input ports configured to receive a corresponding plurality of input signals there through, and (ii) at least one output port, the output port being configured to output an adjusting signal. The corresponding plurality of input signals includes a number of primary signals and two or more reference signals, all of the signals being associated with a first timing phase of operation. The adjusting signal is produced based upon a comparison between the number of primary signals and at least one of the reference signals. Also included is a pair of active devices having gates thereof coupled together, the gates being configured to receive the adjusting signal. The active devices are configured to (i) provide a gain to the adjusting signal in accordance with a predetermined gain factor, and (ii) facilitate an adjustment to the number of primary signals based upon the gain during a second timing phase of operation.

Features and advantages of the present invention include an ability to control or eliminate output common-mode voltage variations in a differential amplifier without the need of providing a tail current source. This approach facilitates the realization of differential amplifiers with larger output swings that are low-power, fast, and operate efficiently.

BRIEF DESCRIPTION OF THE FIGURES

The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate an embodiment of the present invention and, together with the description, explain the purpose, advantages, and principles of the invention. In the drawings:

FIG. 1 is a simplified circuit diagram of a conventional folded cascode differential amplifier;

FIG. 2 is a simplified circuit diagram of a conventional telescopic differential amplifier with a tail current source;

FIG. 3 is a simplified circuit diagram of a conventional telescopic differential amplifier without a tail current source;

FIG. 4 depicts a conventional operational amplifier symbol representing the circuit of FIG. 3;

FIG. 5 depicts the circuit arrangement of FIG. 4 used in a cascode amplifier;

FIG. 6A is an illustration of a conventional switched capacitor circuit arrangement used in a telescopic amplifier without tail current;

FIG. 6B is an illustration of the switched capacitor circuit of FIG. 6A during a first timing phase;

FIG. 6C is an illustration of the switched capacitor circuit of FIG. 6A during a second timing phase;

FIG. 7 is an illustration of a split input transistor technique used in the present invention;

FIG. 8 is schematic diagram of a circuit constructed and arranged in accordance with an embodiment of the present invention;

FIG. 9 depicts a switched capacitor circuit configuration used in the embodiment of FIG. 8;

FIG. 10A is an illustration of exemplary gain factor relationships associated with a first set of split input transistors in the circuit of FIG. 8;

FIG. 10B is an illustration of exemplary gain factor relationships associated with a second set of split input transistors in the circuit of FIG. 8;

FIG. 11 illustrates the operation of the switched capacitor circuit and the split transistors in the embodiment of FIG. 8;

FIG. 12A depicts a first set of switchable capacitors configured for operation during timing phase 1;

FIG. 12B depicts a second set of switchable capacitors configured for operation during timing phase 1;

FIG. 13A depicts the first set of switchable capacitors depicted in FIG. 12A configured for operation during timing phase 2; and

FIG. 13B depicts the second set of switchable capacitors depicted in FIG. 12B configured for operation during timing phase 2.

DETAILED DESCRIPTION OF THE INVENTION

The following detailed description of the present invention refers to the accompanying drawings that illustrate exemplary embodiments consistent with this invention. Other embodiments are possible, and modifications may be made to the embodiments without departing from the spirit and scope of the invention. Therefore, the following detailed description is not meant to limit the invention. Rather, the scope of the invention is defined by the appended claims.

It would be apparent to one of skill in the art that the present invention, as described below, may be implemented in many different embodiments of hardware, software, and/or firmware, in the entities illustrated in the figures. The actual hardware used to implement the present invention is not limiting of the present invention. Thus, the operation and behavior of the present invention will be described with the understanding that modifications and variations of the embodiments are possible, given the level of detail present herein.

FIG. 1 illustrates a conventional folded cascode amplifier 100, having a maximum non-inverting output current 101 of 2I and a maximum inverting output current 102, also equal to 2I, for total current output of 4I. Although the folded cascode amplifier 100 has reasonably high total output current, the amplifier 100 is inefficient. Its slew current, described above, is ±I. The slew current is produced by disconnecting one of the amplifier inputs and measuring the amount of current tilting to, or that can be measured at, the other input. Given a total current of 4I, and the slew current of ±I, the amplifier 100 has an efficiency of about 25%.

Another disadvantage of the folded cascode amplifier 100 is that it is comparatively slow. That is, any input signal provided at inverting input port IN and non-inverting input port IP must travel around a loop 104 prior to amplification. The signal requires time to completely traverse this loop, thereby creating a slower response time for the amplifier 100. However, the folded cascode amplifier 100 has a reasonably high output swing of V_(S)−4V_(dsat). V_(S) is the supply voltage. V_(dsat) is a measure of the source to drain voltage of each of the amplifiers required for each transistor stage to remain in saturation while the amplifier continues to provide full gain.

FIG. 2 is an illustration of a straight telescopic amplifier 200. The amplifier 200 provides a total non-inverting current output 201 of 2I and a total inverting output current 202 of 2I. Thus, the total current of the telescopic amplifier 200 is the same as the folded cascode amplifier 100, 4I, but the slew current is twice as large. The telescopic amplifier 200, however, is faster than the cascode amplifier 100 because it eliminates the signal loop 104 shown in the amplifier 100. Thus, the amplifier 200 consumes the same amount of current as the amplifier 104, but has the advantage of being faster. One disadvantage, however, is that the total output swing of the amplifier 200 is V_(S)−6V_(dsat), lower than that of the output swing of the amplifier 100. As stated above, the amplifier 100 has an output swing of V_(S)−4V_(dsat). Additionally, the amplifier 200 includes a tail current source 208 for controlling common-mode output gain. The tail current source 208 is a fixed bias voltage, which helps reject common-mode voltages. Tail currents are undesirable, however, because of the inherent inefficiencies discussed above.

FIG. 3 shows a telescopic amplifier 300 without a tail current. The amplifier 300 provides a total current output of 4I, equal to the total current output of amplifiers 100 and 200. Further, it does not have the input loop 204 of the amplifier 100, thus providing an operational speed equivalent to the amplifier 200. Yet, the amplifier 300 has an output swing of V_(S)−4V_(dSAT), which is greater than the output swing of the amplifier 200. Thus, the amplifier 300 offers the features and advantages of the folded cascode amplifier 100 and the straight telescopic amplifier 200 without the need for a tail current. However, without a tail current, the amplifier 300 is unable to reject output to common-mode voltages that can be caused by the input source or caused by the previous stage.

FIG. 4 illustrates a conventional use of differential amplifiers, such as the telescopic amplifier 300. More specifically, FIG. 4 shows how the telescopic amplifier 300 can be used to form an operational amplifier 400 having respective inverting and non-inverting input ports IN and IP. Also included are respective inverting and non-inverting differential output ports OP and ON. As known in the art, differential amplifiers operate on the principle that each differential input is sensitive to the opposite polarity of the other. That is, when the non-inverting input receives a positive signal and the inverting input inverts the positive signal to form a negative version thereof, then an output is produced that is equal to a difference between the two signals multiplied by some gain. On the other hand, if both inputs receive a signal having the same value and polarity, an output signal will not be produced, since these two input signals will cancel one another out. Input signals having the same polarity at both inputs are known as common-mode inputs. Thus, it can be said that differential amplifiers have an inherent ability to reject these common-mode signals since the difference between these two signals is zero, causing the signals to cancel one another out.

An inherent common mode rejection ability alone, however, is insufficient to reject common-mode type signals created by offsets or variations in transistor amplification characteristics. That is, transistors forming inverting and non-inverting amplifier input stages may not be precisely matched in their amplification characteristics. One design solution is to offset the characteristics of one transistor to compensate for the amplification characteristics of the other. An undesirable by-product of these offsets is that they are also amplified along with actual input signals, which unnecessarily consumes useful amplifier head-room. For amplifiers having a small amount of gain, such as a gain of 1, the offsets will not create significant problems. However, in telescopic amplifiers without tail currents in a cascoded configuration, such as the operational amplifier 400, offsets are problematic since gains of respective individual amplifiers will be combined.

FIG. 5 shows a conventional cascode amplifier 500 including two amplification stages 502 and 504. The stages 502 and 504 include amplifiers 506 and 508. Each amplifier 506 and 508 has a gain of 4, which produces a total gain of 16 for the amplifier 500. Therefore, the signal level of desirable signals will be multiplied by a factor of 16 during amplification within the amplifier 500. However, unwanted signals produced, for example, by offsets in the amplification stage 502, will also be multiplied by the factor of 16. These amplified unwanted signals (i.e., offsets) often behave as common-mode voltage signals which significantly reduce amplification characteristics, such as dynamic range and head-room, of the amplifier 500. Therefore, although telescopic amplifiers without tail currents inherently reject common-mode voltages, these amplifiers still remain vulnerable to the effects of common-mode type offset voltages.

One conventional technique used to further enhance the common-mode rejection characteristics of telescopic amplifiers is switched capacitor interpolation circuits. Switched capacitor interpolation circuits are routinely used in conjunction with a reference voltage source to further attenuate the effects of output common-mode voltages. As stated above, however, such arrangements are ineffective against offset voltages that may occur as the capacitors of the switched capacitor circuits dump charges from phase to phase transitions, or because of differences in amplification characteristics of the associated transistors. The operation of a conventional telescopic amplifier using a switched capacitor circuit is explained below.

FIGS. 6A-6C illustrate different operational phases of a conventional telescopic amplifier, without tail current, using a switched capacitor circuit. FIG. 6A depicts a telescopic amplifier 600 having respective differential non-inverting and inverting amplification sides 602 and 604 respectively. The non-inverting amplification side 602 includes a transistor T₁ having a gate acting as an inverting input port IN. Also included is a non-inverting output port OP, a current source I₁, a capacitor C₁, and capacitor switches (switch cap circuits) S_(1a) and S_(1b). A current source I₂, providing substantially the same level of current as the current source I₁, provided along with a diode D₁, is configured to provide a diode voltage. The diode D₁, the current source I₂, the capacitor C₁, and the switch cap circuits S_(1a) and S_(1b) are provided as part of the switched capacitor arrangement discussed above. Finally, a voltage source V_(INPUT) is included to provide a reference voltage.

In FIG. 6A, the switch cap circuits S_(1a) and S_(1b) are shown to be in a neutral unswitched configuration. Similar to the non-inverting amplification side 602, the non-inverting amplification side 604 includes a transistor T₂ having a gate forming a non-inverting input port IP, an output port ON, and another current source I₁. Also included in the non-inventing side 604 is another diode D₂, another current source I₂, another capacitor C₂, and capacitor switches S_(2a) and S_(2b). Another reference voltage source V_(INPUT) is provided. In the amplification sides 602 and 604, the switch cap circuits S_(1a) and S_(1b) are provided to respectively connect terminals of the capacitor C₁ to the diode D₁ and the voltage V_(INPUT) during one timing phase, and to switch the terminals to connect to the gate of the transistor T₁ and the output OP during another timing phase. Correspondingly, in the amplification side 604, the switch cap circuits S_(2a) and S_(2b) switch the terminals of the capacitor C₂ from connection with the diode D₂ and the voltage V_(INPUT) during one phase, to connection with the transistor T₂ and the output ON during the other phase.

As known in the art, traditional switch cap circuits operate through two timing phases (i.e., phase 1 and phase 2). Operation of the telescopic amplifier 600 during phase 1 of the switched capacitor circuit is more clearly illustrated in FIG. 6B.

FIG. 6B shows phase 1 of the switched capacitor circuit as noted above. During phase 1, the switches S_(1a) and S_(1b) are configured to provide the voltage V_(INPUT) on one terminal of the capacitor C₁ while connecting the other terminal of the capacitor C₁ to the diode D₁. As part of a known technique to attenuate common-mode output voltage, the diode D₁, which is a field effect transistor (FET) configured as a diode, is provided to mirror the characteristics of transistor T₁. Similarly, the current source I₂ mirrors the current value of the current source I₁ of the amplification side 602. As stated above, the objective of the switch capacitor, the diode D₁, and the current source I₂ is to attenuate common-mode output voltages provided at the output port OP, limiting these voltages to a value substantially equal to the value V_(INPUT).

Therefore, during phase 1, the capacitor C₁ is connected to have the input voltage source V_(INPUT) at one terminal, and the diode D₁ connected at its other terminal. When activated, the current source I₂ produces a voltage Vgs across a gate of diode D₁ as shown in FIG. 6B. Similarly, during phase 1, the switches S_(2a) and S_(2b) are configured to connect the input voltage V_(INPUT) to one terminal of C₂ and connect the diode D₂ to its other terminal as shown. In the circuit of FIG. 6B, the characteristics of the diode D₁ and the transistor T₁ respectively mirror characteristics of the diode D₂ and the transistor T₂.

During phase 1, when the capacitor C₂, which is substantially the same value as the capacitor C₁, is connected to and thereby charged with the input voltage V_(INPUT), the current source I₂ of the amplification side 604 also produces the voltage Vgs across a gate of the diode D₂. Therefore, during phase 1, the capacitor C₁ is charged with a voltage value functionally related to the input voltage V_(INPUT) and the voltage Vgs. Similarly, the capacitor C₂ is charged with a voltage value functionally related to the input voltage V_(INPUT) and the voltage value Vgs, also produced across diode D₂. During phase 2, however, the switched capacitor circuit switches the capacitor C₁ so that the capacitor C₁ is now inserted (i.e., parked) across the output terminal OP and the input terminal IN of the amplification side 602. Similarly, during phase 2, the switches S_(2a) and S_(2b) are configured to park the capacitor C₂ across the output ON and the gate of transistor T₂ of the amplification side 604.

FIG. 6C shows phase 2 of the switched capacitor circuit. As discussed above, during phase 2, the switches S_(1a) and S_(1b) respectively switch the capacitor C₁ so that it is parked across the output OP and the input IN of the input side 602. Similarly, during phase 2 the switches S_(2a) and S_(2b) switch the capacitor C₂, so that it is parked across the output ON and the input IP of the input side 604. Therefore, the voltages that were applied to C₁ during phase 1, V_(INPUT) and Vgs, may now be applied across the output terminal OP and the input terminal IN, respectively. Using this technique, during phase 2 the output voltage provided at the output terminal OP will now be held to the value related to V_(INPUT), Vgs and the value Vgs will be provided at the input port IN, regardless of the presence of any common-mode voltage. The same process applies to the input side 604 with respect to the operation of the capacitor C₂, the switch S_(2a) and the switch S_(2b). This switched capacitor circuit technique creates the effect of attenuating output common-mode voltages that are produced at the outputs OP and ON.

The diode voltage Vgs is used as a reference voltage to control the level of the input and output common-mode voltage associated with the amplification sides 602 and 604 of the amplifier 600. This technique is effective as long as characteristics associated with the diode D₁ carefully match those same characteristics associated with the transistor T₁. Specifics of this technique will be discussed in detail with regard to the amplification side 602 only, although it is understood that this technique applies equally to the amplification side 604.

The gate characteristics associated with the diode D₁ must carefully match those of the gate of the transistor T₁, such that when the current source I₂ is used to set up the diode voltage Vgs across the diode gate, substantially the same voltage Vgs is also produced at the gate of the transistor T₁, which will be a function of the current source I₁. Therefore, when the capacitor C₁ switches from phase 1 to phase 2 and imparts Vgs across the gate of the transistor T₁ and voltage V_(INPUT) across the terminal OP, the output common-mode voltage is held to the value of V_(INPUT). That is, the output common-mode voltage can be attenuated and held to the voltage level V_(INPUT) as long as the characteristics of the diode D₁ gate continue to match those of the gate of transistor T₁. If this relationship holds true, then when the capacitor C₁ changes from phase 1 to phase 2, the diode voltage Vgs will be the same at the diode D₁ gate and the transistor T₁ gate.

Switched capacitor techniques are effective for reducing common-mode voltages when the characteristics of the amplifier's transistors are carefully matched. This technique, however, is not effective when imbalances, offsets, and/or differences in gate characteristics between the diode and the transistor gate exist. In other words, the technique illustrated in FIGS. 6A-6C is only effective at attenuating common-mode voltages in telescopic amplifiers without tail current. Therefore, an improved technique is needed.

In the present application, the inventors have discovered that by building upon the common-mode rejection technique of FIGS. 6A-6C, amplifiers without tail currents can be provided with techniques that not only attenuate common-mode voltages, but also compensate for the effects of common-mode offsets.

FIG. 7 illustrates, among other things, that any traditional amplifier without a tail current, such as the telescopic amplifier 300 shown in FIG. 3, can be modified to compensate for the effects of common-mode offsets. A first step in this modification requires that the amplifiers' input stage transistors, 700 and 702, be divided into transistor pairs 704 and 706 respectively. As such, the transistor 700 is shown to be divided into transistors 700 a and 700 b. That is, the gain characteristics associated with the single transistor 700 are divided between the two transistors 700 a and 700 b of the transistor pair 704. Further, the gain characteristics of transistor 702 are divided between the transistors 702 a and 702 b of the transistor pair 706. As will be described in greater detail below, the transistor pairs 702 and 704 may be used in conjunction with other circuit components to carefully control the input and output common-mode voltages associated with traditional amplifiers that do not have tail currents. FIG. 8 illustrates an exemplary implementation of the split transistor technique shown in FIG. 7.

In FIG. 8, an exemplary amplifier 800 is constructed and arranged in accordance with a preferred embodiment of the present invention. The amplifier 800 is a modified telescopic differential amplifier, although the present invention may be used with any differential amplifier that does not have a tail current. In the exemplary amplifier 800, the transistors are metal oxide semiconductor field-effect transistor (MOSFET) devices of the N-channel (NMOS) variety. The present invention, however, is not limited to NMOS transistors. In FIG. 8, the amplifier 800 includes respective first and second amplification sides 801 and 802 respectively. The amplification side 801 includes an amplifier inverting input stage 804 having a signal input transistor 805 with its gate forming an input port labeled “IN” and a companion transistor 806. Also included in the input side 801, are transistors 808-810 representing individual amplification stages. A non-inverting output port labeled “OP” is provided to output a differential signal.

Similarly, the amplification side 802 includes a non-inverting amplifier input stage 812 having an input transistor 814 with its gate forming a non-inverting input port labeled “IP” along with a companion transistor 815. Gates g_(n) and g_(p) of the respective companion transistors 806 and 815, are connected together. Also included in the amplification side 802 are amplification stage transistors 818-820. The amplifier 800 includes a first voltage source 822 for providing a reference adjustment voltage V_(b4r) and a current source 824 providing a current I. Also included is a second voltage source 826 for providing a signal representative of a desired fixed common-mode voltage level V_(cm), and a supply voltage source V_(S). Common nodes 828 and 829 are provided to electrically connect the amplification sides 801 and 802 and their associated components.

As readily observed from FIG. 8, the input stages 804 and 812 are constructed of differential pair transistors respectively formed from transistor pairs 805, 806 and 814, 815. As explained above, the differential pair transistors 805, 806 and 814, 815 may be formed by splitting input stage transistors, such as the transistors 700 and 702 shown in FIG. 7. The idea is to divide the gain characteristics of the single transistor, in a conventional differential amplifier, across the transistor pair. In the exemplary embodiment of FIG. 8, the sources of transistors 805, 806, 814 and 815 are all connected to the common node 829. Additionally, one end of the voltage source 826 and an anode side of the diode voltage source 822 are also connected to the common node 829. As shown, the gates g_(n) and g_(p) of transistors 806 and 815 are coupled to a connecting line L1 and are both structured to receive as an input a correcting voltage signal V_(b4).

Also as shown in FIG. 8, drains of the transistors 805 and 806 are connected together and are coupled to the source of the transistor 810. Similarly, the drains of transistors 814 and 815 are connected together and are coupled to the source of the transistor 820. The supply voltage V_(S) is provided to the common node 828 to provide an operational voltage to the amplifier 800.

Finally, the amplifier 800 includes a common-mode feedback circuit 830, provided in cooperative arrangement with the output ports OP and ON and the input stages 804 and 812. The common-mode feedback circuit 830 cooperates with the amplifier input stages 804 and 812 to provide a feedback loop to attenuate the effects of common-mode offset voltages produced at the output ports OP and ON. The feedback circuit 830 includes input ports 833 and 834 for respectively receiving signals produced at the output ports OP and ON. Also included are input ports 831 and 832 for respectively receiving the voltage signals V_(cm) and V_(b4r). Finally, an output port 836 provides the correcting voltage V_(b4) along the output line L₁. The voltage V_(b4) is injected into the gates g_(n) and g_(p) of the transistors 806 and 815 respectively. More specifically, the common-mode feedback circuit 830 monitors the actual output common-mode voltage provided at the output ports OP and ON. The feedback circuit 830 then compares the actual common-mode voltage occurring at the ports OP and ON with the fixed desirable voltage V_(cm) and correspondingly adjusts the common-mode output voltage, using V_(b4) to change the output common-mode voltage to the desirable voltage level.

The common-mode feedback circuit 830 is known in the art as a switched capacitor circuit. Thus, the common-mode feedback circuit 830 includes two sets of capacitors that are alternately switched between the various input and output ports shown in FIG. 8. The input stages 804 and 812 are configured to adjust an input common-mode voltage based upon an output from the common-mode feedback circuit 830 in a predetermined manner. The common-mode feedback circuit 830 is described more fully with reference to the illustration of FIG. 9. Similarly, the operation of the input stages 804 and 812 will be described more fully in reference to the illustrations shown in FIGS. 10A and 10B.

FIG. 9 is a schematic diagram of switchable capacitors that are included in the common-mode feedback circuit 830. The switched capacitor circuit 830 includes respective first and second capacitor sets 902 and 904. During operation of the amplifier 800, the capacitor sets 902 and 904 are switched between the various amplifier inputs and outputs in a ping-pong like fashion. In this manner, at least one set of capacitors always remains connected across the amplifier inputs and outputs to avoid the inadvertent creation of an open loop circuit condition.

In particular, the first set of capacitors 902 includes respective first and second capacitors C_(1a) and C_(2b) and associated switches S₁-S₃ for switching the capacitors C_(1a) and C_(2a) between first and second timing phases. For example, the capacitors C_(1a) and C_(2a) are switched from a connection to the output ports OP, V_(b4), and ON during a first timing phase of operation, to a connection to V_(cm) and V_(b4) during a second timing phase. Similarly, the second set of capacitors 904 includes individual capacitors C_(1b) and C_(2b) as well as switches S₄-S₆ for alternately connecting the capacitors C_(1b) and C_(2b) between the first and second timing phases discussed above. In the exemplary embodiment of FIG. 9, the capacitors C_(1b) and C_(2b) are switched from the output ports V_(cm) and V_(b4r) during phase 1, to the output ports associated with OP, V_(b4), and ON, during phase 2.

FIGS. 10A and 10B provide simplified illustrations of the input stages 804 and 812 depicted in FIG. 8. FIGS. 10A and 10B, with reference to FIG. 7 discussed above, illustrate how transistor input amplification characteristics are divided between transistors 805, 806, 814, and 815. As shown from the illustration of FIG. 8, the gates g_(n) and g_(p) of the companion transistors 806 and 815 are coupled to the output port 836 of the common-mode feedback circuit 830. Connected in this manner, the companion transistors 806 and 815 are configured to receive the adjusting voltage V_(b4), produced by the common-mode feedback circuit 830. Having received the correcting voltage V_(b4), the companion transistor 806 and 815 can then adjust the common-mode voltage level, measured at the output ports OP and ON, by a predetermined amount.

A characteristic unique to common-mode voltage signals is that these voltages possess the same polarity on both the input and output sides of amplifiers. On this basis, undesirable common-mode voltage signals can be distinguished from differential-mode voltage signals. Consequently, common-mode voltage signals and differential-mode voltage signals can be separately processed by the amplifier. In the exemplary embodiment of FIG. 8, the differential-mode voltage signals are not attenuated. Only the common-mode voltage signals are attenuated in the present embodiment. In FIG. 8, the transistors 805 and 814 are configured to amplify an input differential-mode signal in accordance with a predetermined gain value. The companion transistors 806 and 815, however, based upon the output from the common-mode feedback circuit 830, will attenuate the signal level to a lower value by the predetermined gain value.

Thus, the common-mode feedback circuit 830 and the amplification stages 804 and 812 form a feedback path to sample the output common-mode voltage at OP and ON, compare the sampled voltage with the voltage V_(cm) and produce the correcting signal V_(b4) as a result. The companion transistors 806 and 815 receive this adjusting voltage signal V_(b4) and adjust the voltage level of the output common-mode signal by the predetermined amount. This process, explained in greater detail below, carefully controls the output common-mode voltage. The present invention is thus configured to lower the output common-mode voltage level when the common-mode output voltage is too high, based upon reference voltage V_(cm). Alternatively, the present invention is also configured to raise the output common-mode voltage level when the output common-mode voltage is lower than V_(cm).

In FIGS. 10A and 10B, the transistor pairs 805/814 and 806/815 are structured to provide complimentary gain values in order to process the input and output differential and common-mode voltage signals. More specifically, the transistor pairs 805/814 and 806/815 operate in accordance with respective gain factors 1002 and 1004. For purposes of illustration only, ¾x and ¼x have been respectively chosen for the factors 1002 and 1004. It should be understood, however, that any factors may be chosen as long as their sum is substantially equal to one (e.g., ⅖ & ⅗, ½ & ½, and ⅓ & ⅔ etc.). The gain factor 1002 and the corresponding transistors 805 and 814 are used to provide amplification for desired differential-mode signals received at the input ports IN and IP respectively. On the other hand, the gain factor 1004 and associated transistors 806 and 815 is configured to compliment the operation of transistors 805 and 814 to adjust the output common-mode voltage. Specifically, the transistors 806 and 815 are used to either attenuate or amplify the common-mode voltage measured at OP and ON based upon a comparison with the fixed level voltage signal V_(cm). This principle is illustrated more clearly in the example of FIG. 11.

FIG. 11 explains the operation of the present invention. In FIG. 11, when output signals are delivered to the output ports OP and ON of the amplifier 800, they are also respectively provided as inputs to the input ports 833 and 834 of the common-mode feedback circuit 830. When received at the input ports 833 and 834, an actual common-mode voltage level of these signals is measured and compared with the desirable common-mode voltage signal V_(cm) supplied by the voltage source 826. In practice, the capacitors C_(1a) and C_(2a) hold an amount of charge acquired during a previous phase, based on the voltage V_(cm). When the capacitors C_(1a) and C_(2a) are connected across the outputs OP and ON, their charge voltage, substantially equal to V_(cm), pulls the correcting voltage V_(b4), up or down, based upon the level of the actual common-mode voltage present in the output signals received at the ports 833 and 834.

If the actual output common-mode voltage is higher than V_(cm) during phase 1, for example, as indicated by the up arrows a_(p1) at the input ports 833 and 834, then the common-mode feedback circuit 830 reacts to adjust the output common-mode voltage level. Specifically, if the common-mode voltage at the input port 833 and 834 is too high, the common-mode feedback circuit 830 produces a correcting signal S_(C). S_(C) has the voltage level of V_(b4) and is produced to provide the needed adjustment to the common-mode output voltage level. The correcting signal S_(C) is injected into the gates g_(n) and g_(p) of the transistors 806 and 815.

The transistors 806 and 814 behave as inverting amplifiers. Therefore, in response to the gates g_(n) and g_(p) going up, during phase 2, the common-mode voltage produced at the outputs OP and ON, is correspondingly drawn down, as represented by the arrows a_(P2). Thus, whenever the output common-mode voltage at the output ports OP and ON is higher than V_(cm) during one phase, the common-mode feedback circuit 830 and the transistors 806 and 815 cooperate to reduce the common-mode voltage during the next phase.

On the other hand, if the common-mode output voltage is lower than V_(cm), then the common-mode feedback circuit 830 and the transistors 806 and 815 cooperate to increase the output common-mode voltage in association with the gain factor 1004. During the next phase, input signals received at the input ports IN and IP are amplified based upon the gain factor 1002. Thus in the exemplary embodiment of FIG. 11, the input differential-mode signals will be amplified by a nominal gain moderated by the factor ¾x. The present invention, therefore, provides a feedback loop operating between phase 1 and phase 2 of the switch capacitor circuitry to dynamically adjust the output common-mode voltage. In this manner, the effects of common-mode type voltage offsets can be minimized.

FIGS. 12A-13B provide a more detailed illustration of the operation of the common-mode feedback circuit 830 and the input stages 804 and 812. Although discussed above, with respect to FIGS. 6A-6C, the operation of switch capacitor circuits will be repeated in this section for purposes of review. First, referring briefly to FIG. 9, the switch capacitor circuits are configured to operate during timing phases 1 and 2. As shown in FIG. 9 the capacitors C_(1a), C_(2a), C_(1b), and C_(2b), and the switches S₁-S₆, are ping-ponged in and out of the common-mode feedback circuit 830 in accordance with the phases 1 and 2.

During phase 1, all of the capacitors are configured as shown in FIGS. 12A-12B. That is the capacitors C_(1b) and C_(2b) are configured for connection between the input ports 831 and 832. As shown in FIG. 8, the desirable voltage V_(cm) is injected into the common-mode feedback circuit 830 through the input port 831. Similarly, the reference voltage V_(b4r) is injected in to the common-mode feedback circuit 830 through the input port 832. As stated above, the reference voltage V_(b4r) is generated by the diode voltage source 822 and is provided as the proper reference voltage level needed to precisely adjust the actual output common-mode voltage to match the desired voltage level V_(cm). Therefore, during phase 1, the capacitors C_(1b) and C_(2b) sample, or are charged in accordance with, a voltage V_(X), which is a function of the voltages V_(cm) and V_(b4r) as measured across capacitors C_(1b) and C_(2b).

Also during phase 1, the capacitors C_(1a) and C_(2a) are configured as shown in FIG. 12B. That is, capacitor C_(1a) is connected between the output port OP and the correcting voltage source V_(b4), while the capacitor C_(2a) is connected between the output port ON and the correcting voltage V_(b4). Thus, during phase 1, the capacitors C_(1a) and C_(2a) cooperate to compare the actual common-mode voltage, defined as (V_(A)+V_(B))2, with the voltage V_(X). The voltage V_(X) is substantially equal to the desired common-mode voltage V_(cm). This process occurs as the capacitors C_(1a) and C_(2a) discharge across the terminals OP and ON. At this same time, by discharging across OP and ON, the capacitors C_(1a) and C_(2a) correspondingly pull the value of V_(b4) up or down, based upon the difference between (V_(A)+V_(B))/2 and V_(X). The adjusting voltage V_(b4) is then injected into the gates g_(n) and g_(p) of the transistors 806 and 815 respectively. The gain factor 1004 is then applied to V_(b4) and in-turn reflected in the common-mode output voltage during the ensuing phase.

In other words, the present invention provides at least two sets of capacitors, insuring that at least one set is parked across the amplifier at any given point in time. While one set of capacitors compares the desired common-mode voltage signal V_(cm) with the reference voltage signal V_(b4r) and charges the capacitors accordingly, another set of capacitors samples the actual output common-mode voltage (V_(A)+V_(B))/2 and produces a correcting voltage V_(b4). While one set of capacitors is sampling and comparing the other set of capacitors is discharging and correcting.

As stated above, during phase 1 the capacitors C_(1b) and C_(2b) respectively sample the desirable common-mode output voltage signal V_(cm) and the reference voltage signal V_(b4r). During phase 2, however, as shown in FIG. 13A, the capacitors C_(1b) and C_(2b), previously connected between V_(cm) and V_(b4r), are switched to respectively connect across the outputs OP and ON and the adjusting voltage V_(b4). That is during phase 1 capacitors C_(1b) and C_(2b) were charged by voltage V_(X), a function of V_(cm) and V_(b4r). During phase 2, however, the capacitors C_(1b) and C_(2b) discharge and inject the adjusting voltage V_(b4) into the gates g_(n) and g_(p) of the respective transistors 806 and 815. Therefore, using capacitors C_(1b) and C_(2b) as an example, during phase 1 the capacitors are loaded with the correct common-mode voltage level having V_(cm) on one side, and the reference voltage V_(b4r) on the other side.

During phase 2, as shown in FIG. 13A, the capacitors discharge the adjusting voltage V_(b4) and inject V_(b4) into the gates g_(n) and g_(p) of the transistors 806 and 815. The parasitic capacitance values of these gates are adjusted in accordance with the gain adjustment factor 1004 shown in FIG. 11. The output common-mode voltage levels are then correspondingly pulled down, or up, in accordance with this value.

During the ensuing phase 1, the desirable voltage level signal V_(cm) is again compared with the actual output common-mode voltage (V_(A)+V_(B))/2 as seen at the output terminals OP and ON. If, for example, the actual common-mode voltage is equal to V_(cm) and the reference voltage V_(b4r) is equal to the correcting voltage V_(b4), then no adjustment will be made. If, however, the actual output common-mode voltage is higher than V_(cm), then the fixed voltage V_(X) across the capacitors C_(1b) and C_(2b) will then pull the middle node V_(b4), correspondingly higher. That is, if the actual output common-mode voltage is higher than the fixed voltage V_(X), which is substantially equal to V_(cm), the fixed voltage V_(X) is analogous to placing a battery between each of the terminals 833 and 834 and the output node 836. Therefore, the fixed voltage, behaving as a battery, will also pull the correcting voltage V_(b4) correspondingly higher, which will in turn pull the output common-mode voltage of the amplifier correspondingly lower, since the transistors 806 and 815 act as inverting amplifiers. As previously stated, the capacitors C_(1b) and C_(2b) are ping-ponged between the common-mode feedback circuit 830 in accordance with phase 1 and phase 2.

Therefore the aforementioned process switches between the capacitors C_(1b) and C_(2b) and the capacitors C_(1a), and C_(2a) during the respective phases. Thus, during phase 2, as shown in FIG. 13A, while C_(1b) and C_(2b) are discharging the adjusting voltage V_(b4), the capacitors C_(1a) and C_(2a) are sampling and comparing the actual common-mode voltage, as seen on the output terminals OP and ON, with the desirable common-mode voltage V_(cm). Using the aforementioned structure and technique, the present invention provides an accurate and reliable circuit and method to dynamically sample the common-mode output voltage of an amplifier without tail current compare this output common-mode voltage with a reference signal and adjust the common-mode input voltage in a manner to dynamically adjust the output common-mode voltage to the desirable level. This circuit arrangement and technique therefore creates the ability to compensate for the effects of common-mode type offset voltages caused by variations in electrical characteristics associated with amplifier transistors. The circuit arrangement rejects input common-mode voltages created by the input source or by a previous amplification stage.

The foregoing description of the preferred embodiments provide an illustration and description, but is not intended to be exhaustive or to limit the invention to the precise form disclosed. Modifications or variations are possible consistent with the above teachings or may be acquired from practice of the invention. Thus, it is noted that the scope of the invention is defined by the claims and their equivalents. 

What is claimed is:
 1. An apparatus comprising: a switched capacitor feedback circuit including (i) a plurality of input ports configured to receive a corresponding plurality of input signals therethrough, and (ii) at least one output port, the output port being configured to output an adjusting signal; wherein the corresponding plurality of input signals includes a number of primary signals and two or more reference signals, all of the signals being associated with a first timing phase of operation; and wherein the adjusting signal is produced based upon a comparison between the number of primary signals and at least one of the reference signals; a pair of active devices having gates thereof coupled together, the gates being configured to receive the adjusting signal; wherein the active devices are configured to (i) provide a gain to the adjusting signal in accordance with a predetermined gain factor, and (ii) facilitate an adjustment to the number of primary signals based upon the gain during a second timing phase of operation.
 2. The apparatus of claim 1, wherein the switched capacitor feedback circuit includes at least two pairs of capacitors, capacitors of each pair being connected together by configurable switches; and wherein the switches are configured to switch the pairs of capacitors in accordance with the first and second timing phases.
 3. The apparatus of claim 1, wherein the apparatus is an amplifier; wherein the primary signals are amplifier output signals; and wherein the two or more reference signals include a fixed level voltage signal having a desirable voltage level and a reference voltage signal having a voltage level substantially equal to a difference between a voltage level of the amplifier output signals and the fixed level voltage signal.
 4. The apparatus of claim 1, wherein the active devices are transistors.
 5. The apparatus of claim 4, wherein the transistors are N-channel metal oxide semiconductor field effect transistors.
 6. The apparatus of claim 1, wherein the amplifier is of a telescopic variety and excludes a tail current source.
 7. A common-mode voltage adjusting circuit configured for cooperative arrangement with (i) a common-mode feedback module including switchable capacitors, and (ii) a differential amplifier, the amplifier having at least inverting and non-inverting input stages including respective first and second active devices, gates of the first and second active devices respectively forming inverting and non-inverting input ports and sources thereof coupled to a common amplifier node, the common-mode voltage adjusting circuit comprising: third and fourth active devices having gates thereof coupled together to form an adjusting signal input node and sources thereof coupled to the common amplifier node; wherein drains of the first and third active devices are connected together and drains of the second and fourth active devices are connected together; wherein during a first timing phase of operation the common-mode feedback module is configured to (i) receive output signals produced by the amplifier, (ii) receive a number of reference signals produced by reference voltage sources, (iii) compare the output signals with the number of reference signals, and (iv) produce an adjusting signal based upon the comparison; and wherein each of the third and fourth active devices is configured to (i) receive the adjusting signal via the adjusting signal input node, (ii) provide a gain to the adjusting signal in accordance with a first predetermined gain factor, and (iii) facilitate an adjustment to one or more of the number of the output signals based upon the gain during a second timing phase of operation.
 8. The common-mode voltage adjusting circuit of claim 7, wherein the common-mode feedback module includes at least two pairs of switchable capacitors connected together by configurable switches; and wherein the switches are configured to switch the pairs of capacitors in accordance with the first and second timing phases.
 9. The common-mode voltage adjusting circuit of claim 7, wherein the number of reference signals includes a fixed level voltage signal having a desirable voltage level and a reference voltage signal having a voltage level substantially equal to a difference between a voltage level of the output signals and the fixed level voltage signal.
 10. The common-mode voltage adjusting circuit of claim 7, wherein the active devices are transistors.
 11. The common-mode voltage adjusting circuit of claim 10, wherein the transistors are N-channel metal oxide semiconductor field effect transistors.
 12. The common-mode voltage adjusting circuit of claim 7, wherein the amplifier is of a telescopic variety and excludes a tail current source.
 13. The common-mode voltage adjusting circuit of claim 7, wherein the adjustment to one or more of the output signals adjusts a common-mode voltage thereof.
 14. The common-mode voltage adjusting circuit of claim 13, wherein the input signals to the amplifier are differential-mode voltage signals.
 15. The common-mode voltage adjusting circuit of claim 7, wherein the first and second active devices are configured to receive respective input signals via the inverting and non-inverting input ports, each of the first and second active devices being structured and arranged to provide a gain to its respective input signal in accordance with a second predetermined gain factor.
 16. The common-mode voltage adjusting circuit of claim 15, wherein the first and second predetermined gain factors are related.
 17. The common-mode voltage adjusting circuit of claim 16, wherein a sum of the first and second predetermined gain factors is substantially equal to one.
 18. The common-mode voltage adjusting circuit of claim 17, wherein one of the first and second predetermined gain factors is ¼th and the other of the first and second predetermined gain factors is ¾ths.
 19. An amplifier comprising: an input stage including two or more input transistors, gates thereof forming respective input ports, drains thereof being respectively connected, at least indirectly, to amplifier output ports, and sources thereof being connected to a common node; wherein the input transistors are configured to (i) receive respective input signals via the input ports, (ii) multiply the received input signals in accordance with a first predetermined gain factor, and (iii) facilitate production of respective output signals based upon the multiplied input signals; and a feedback device including two or more feedback transistors, gates of the feedback transistors being connected together to form a feedback input node, sources thereof being connected to the common node, and drains thereof being connected to the drains of the input transistors; wherein the feedback transistors are configured to (i) receive respective adjusting signals via the feedback input node, (ii) multiply the respective adjusting signal in accordance with a second predetermined gain factor, and (iii) adjust the respective output signals based upon the multiplied adjusting signal.
 20. The amplifier of claim 19, wherein the amplifier is a differential amplifier; and wherein the respective input ports are inverting and non-inverting input ports.
 21. The amplifier of claim 19, wherein the first and second predetermined gain factors are related.
 22. The amplifier of claim 21, wherein a sum of the first and second predetermined gain factors is substantially equal to one.
 23. The amplifier of claim 22, wherein one of the first and second predetermined gain factor is ¼th and the other of the first and second predetermined gain factors is ¾ths.
 24. The amplifier of claim 23, wherein all of the transistors are N-channel metal oxide semiconductor field effect transistors.
 25. A differential amplifier comprising: first, second, and third pairs of transistors, sources of the first pair being connected to a first common node, drains of the first pair being connected to sources of the second pair, and drains of the second and third pairs being connected together; an inverting input transistor having (i) its drain connected to a source of one transistor of the third pair, (ii) its source connected to a second common node, and (iii) its gate forming an inverting input port; wherein the drain of the one transistor of the third pair forms a non-inverting output port; a non-inverting input transistor having (i) its drain connected to a source of the other transistor of the third pair, (ii) its source connected to the second common node, and (iii) its gate forming a non-inverting input port; wherein the drain of the other transistor of the third pair forms an inverting output port; a pair of feedback transistors having gates thereof connected together to form an adjusting signal input node, drains thereof respectively connected to the drains of the inverting and non-inverting input transistors, and sources thereof connected to the second common node; a reference voltage source configured to provide a reference voltage signal, the reference voltage source having a negative end connected to the second common node and a positive end connected to the first common node; a common-mode voltage source configured to produce a desirable common-mode voltage signal and having a first end connected to the second common node; and a common-mode voltage feedback module including two pairs of capacitors, the two pairs of capacitors being switchable between two or more timing phases of operation, the common-mode voltage feedback module being configured to (i) receive output signals produced by the amplifier, (ii) receive the reference voltage signal and the desirable common-mode voltage signal, (iii) compare the output signals with the reference voltage signal and the desirable common-mode voltage during a first of the two or more timing phases, and (iv) produce an adjusting signal based upon the comparison during a second of the two or more timing phases, the adjusting signal being applied to the adjusting signal input node to adjust the output signals.
 26. The differential amplifier of claim 25, further comprising a supply voltage source connected to the first common node and configured to provide a supply voltage.
 27. The differential amplifier of claim 26, further comprising a current supply disposed between the reference voltage source and the first common node.
 28. The differential amplifier of claim 27, wherein the differential amplifier is a telescopic amplifier.
 29. The differential amplifier of claim 25, wherein the inverting and non-inverting input transistors are configured in accordance with a first predetermined gain factor; and wherein the feedback transistors are configured in accordance with a second predetermined gain factor.
 30. The differential amplifier of claim 29, wherein the first and second predetermined gain factors are related.
 31. The differential amplifier of claim 30, wherein a sum of the first and second predetermined gain factors is substantially equal to one.
 32. The differential amplifier of claim 31, wherein one of the first and second predetermined gain factors is ¼th and the other of the first and second predetermined gain factors is ¾ths.
 33. The differential amplifier of claim 25, wherein the reference voltage source is a field effect transistor configured as a diode.
 34. The differential amplifier of claim 33, wherein all of the transistors are N-channel metal oxide semiconductor field effect transistors.
 35. A method for adjusting output common-mode voltages in an amplifier, the amplifier including at least a common-mode feedback module having input ports coupled to amplifier output ports and having an output port coupled to gates of a first pair of transistors, the common-mode feedback module being configured to switch between two or more timing phases of operation, gates of the first pair of transistors being connected together to form an adjusting signal input node, drains of the first pair of transistors being connected to drains of a second pair of transistors, and gates of the second pair of transistors forming amplifier input ports, the method comprising: receiving a number of reference voltage signals and output signals as inputs in the common-mode feedback module; comparing the reference voltage signals with the output signals and producing an adjusting signal thereby, the comparing occurring during a first of the two or more timing phases; receiving the adjusting signal in the first pair of transistors via the adjusting signal input node; applying a gain to the adjusting signal in the first pair of transistors, the gain being based upon a first predetermined gain factor; and adjusting the output signals in accordance with the applying during a second of the two or more timing phases.
 36. The method of claim 35, further comprising receiving respective input signals at the amplifier input ports, the second pair of transistors applying a gain to the respective input signals in accordance with a second predetermined gain factor.
 37. The method of claim 36, wherein the first and second predetermined gain factors are related.
 38. The method of claim 37, wherein a sum of the first and second predetermined gain factors is substantially equal to one.
 39. The method of claim 38, wherein one of the first and second predetermined gain factors is ¼th and the other of the first and second predetermined gain factors is ¾ths.
 40. An apparatus comprising: a feedback circuit including two or more capacitors and being configured to (i) receive first and second reference signals as first inputs during a first phase, the first and second reference signals cooperating to produce a particular voltage across the capacitors, (ii) receive an information signal during a second phase, and (iii) provide a correcting signal as an output, the correcting signal being responsive to the information signal; wherein (i) a common-mode voltage of the information signal is compared with the particular voltage and (ii) the correcting signal is produced based upon the comparison.
 41. The apparatus of claim 40, wherein the first reference signal is representative of a desirable common-mode voltage level and the second reference signal is representative of adjustments to the common-mode voltage of the information signal; and wherein the particular voltage is representative of a voltage across the capacitors.
 42. The apparatus of claim 41, wherein the feedback circuit is a switched-capacitor circuit.
 43. The apparatus of claim 42, wherein the correcting signal is produced during the second phase.
 44. The apparatus of claim 43, wherein the capacitors are charged in accordance with the voltage across the capacitors during the first phase; and wherein the comparison is a function of the capacitors discharging during the second phase, the discharge adjusting a voltage level of the correcting signal.
 45. The apparatus of claim 40, wherein the apparatus is an amplifier; and wherein the amplifier is of a telescopic variety and excludes a tail current source. 